Abstract:

Exchange-coupled magnetic multilayer structures for use with toggle MRAM
devices and the like include a tunnel barrier layer (108) and a synthetic
antiferromagnet (SAF) structure (300) formed on the tunnel barrier layer
(108), wherein the SAF (300) includes a plurality (e.g., four or more) of
ferromagnetic layers (302, 306, 310, 314) antiferromagnetically or
ferromagnetically coupled by a plurality of respective coupling layers
(304, 308, 312). The microcrystalline texture of one or more of the
ferromagnetic layers is reduced to substantially zero as measured from
X-Ray Diffraction by exposure of various layers to oxygen, by forming a
detexturing layer, by adding oxygen during the ferromagnetic or coupling
layer fabrication, and/or by using amorphous materials.

Claims:

1. A method for forming an exchange-coupled magnetic multilayer structure,
comprising:providing a first ferromagnetic layer;forming a first coupling
layer on the first ferromagnetic layer;forming a second ferromagnetic
layer on the first coupling layer;forming a second coupling layer on the
second ferromagnetic layer;forming a third ferromagnetic layer on the
second coupling layer;forming a third coupling layer on the third
ferromagnetic layer; andforming a fourth ferromagnetic layer on the third
coupling layer;wherein at least one of the second, third and fourth
ferromagnetic layers has a substantially zero microcrystalline texture
prior to the subsequent forming step.

2. The method of claim 1, wherein the substantially zero microcrystalline
texture is characterized by a rocking curve full-width-at-half-maximum
(FWHM) greater than approximately 10.degree..

3. The method of claim 1, wherein the substantially zero microcrystalline
texture is produced by oxygen treatment of at least one of the coupling
layer.

4. The method of claim 1, wherein the substantially zero microcrystalline
texture is produced by forming a detexturing layer that reduces the
texture of subsequent layers.

5. The method of claim 4, wherein the detexturing layer comprises Al.

6. The method of claim 1, wherein at least one of the first, second, third
and fourth ferromagnetic layers are amorphous layers.

7. The method of claim 1, wherein forming the first coupling layer
includes forming a layer of Ru.

8. The method of claim 1, including forming a total of N ferromagnetic
layers such that N-2 of the ferromagnetic layers have a substantially
zero microcrystalline texture.

9. The method of claim 8, wherein each of the N-2 ferromagnetic layers are
formed to have substantially zero microcrystalline texture due to: the
addition of at least one detexturing layer, the use of an amorphous
layer, the exposure of the coupling layer to oxygen, or addition of
oxygen during ferromagnetic or coupling layer fabrications.

10. A an exchange-coupled magnetic multilayer structure comprising:a
plurality of ferromagnetic layers antiferromagnetically or
ferromagnetically coupled by a plurality of respective coupling layers,
wherein the plurality of ferromagnetic layers includes a first
ferromagnetic layer, a second ferromagnetic layer, a third ferromagnetic
layer and a fourth ferromagnetic layer, wherein at least one of the
second, third, and fourth ferromagnetic layers has a substantially zero
microcrystalline texture.

11. The structure of claim 10, wherein the substantially zero
microcrystalline texture is characterized by a rocking curve
full-width-at-half-maximum (FWHM) greater than approximately 10.degree..

12. The structure of claim 10, wherein the substantially zero
microcrystalline texture is produced by oxygen treatment of at least one
of the coupling layers.

13. The structure of claim 10, wherein the substantially zero
microcrystalline texture is produced by forming a detexturing layer that
reduces the texture of subsequent layers.

15. The structure of claim 10, wherein at least one of the first, second,
third and fourth ferromagnetic layers are amorphous layers.

16. The structure of claim 10, wherein the first coupling layer comprises
a layer of Ru.

17. The structure of claim 10, wherein the plurality of ferromagnetic
layers includes N ferromagnetic layers, and wherein N-2 of the
ferromagnetic layers have a substantially zero microcrystalline texture.

18. The structure of claim 10, wherein each of the N-2 ferromagnetic
layers have a substantially zero microcrystalline texture as the result
of: the addition of at least one detexturing layer, the use of an
amorphous layer, the exposure of the coupling layer to oxygen, or
addition of oxygen during the ferromagnetic or coupling layers
fabrication.

19. A toggle MRAM device comprising:a first electrode;a fixed layer
synthetic antiferromagnet (SAF) formed on the first electrode;a tunneling
barrier formed on the fixed layer SAF;a free layer exchange-coupled
magnetic multilayer structure formed adjacent the tunnel barrier layer,
wherein the exchange-coupled magnetic multilayer structure comprises a
plurality of ferromagnetic layers antiferromagnetically or
ferromagnetically coupled by a plurality of respective coupling layers,
wherein the plurality of ferromagnetic layers includes a first
ferromagnetic layer adjacent the tunnel barrier layer, a second
ferromagnetic layer, a third ferromagnetic layer and a fourth
ferromagnetic layer, wherein at least one of the first, second, third and
fourth ferromagnetic layers has a substantially zero microcrystalline
texture;a cap layer formed on the free layer SAF; anda second electrode
formed on the cap layer.

20. The toggle MRAM of claim 19, wherein the plurality of ferromagnetic
layers includes N ferromagnetic layers, and wherein N-2 of the
ferromagnetic layers has a substantially zero microcrystalline texture.

[0002]Magnetoresistive random access memory (MRAM) technology combines
magnetoresistive components with standard silicon-based microelectronics
to achieve non-volatility, high-speed operation, and excellent read/write
endurance. In a standard MRAM device, information is stored in the
magnetization directions of free magnetic layer in individual magnetic
tunnel junctions (MTJ). Referring to FIG. 1, an MTJ 100 generally
includes a tunneling barrier 108 between two ferromagnetic layers: free
ferromagnetic layer 106, and fixed ferromagnetic layer 110. Each layer
106 and 110 may comprise multiple ferromagnetic layers (a synthetic
antiferromagnet, or "SAF") or a single layer. The fixed layer is
typically formed over a pinning layer 120. The structure is typically
formed over a seed layer 112 and includes a cap layer 130 over the free
layer, and is positioned between two electrodes 102 and 114.

[0003]In a standard MRAM, the bit state is programmed to a "1" or "0"
using applied magnetic fields generated by currents flowing along two
programming lines. The applied magnetic fields selectively switch the
magnetic moment direction of free layer 106 for the bit at the
intersection of two programming lines as needed to program the bit state.
When the magnetic moment directions of free layer 106 and fixed layer 110
are aligned in the same direction, and a voltage is applied across MTJ
100, a lower resistance is measured than when the magnetic moment
directions of layers 106 and 110 are set in opposite directions.

[0004]For toggle MRAM devices, free layer 106 may consist of a standard
SAF as shown in FIG. 2, wherein two ferromagnetic layers 202 and 206 are
antiferromagnetically coupled via a coupling layer 204. Magnetization
directions are shown by the arrows in layers 202 and 206. Tunneling
barrier 108 may comprise a variety of dielectric materials and may have
any suitable structure. In one embodiment, for example, tunneling barrier
layer 108 comprises an aluminum oxide (AlOx layer) having a
thickness of about 6-15 Å.

[0005]The switching field (Hsw) necessary for a toggle transition in
a toggle MRAM is related to the magnetic properties of the patterned SAF
free layer according to the relationship Hsw= {square root over
(HkHsat)}, where Hk is the anisotropy field of the two
ferromagnetic layers in the SAF and Hsat is the saturation magnetic
field of the SAF, and the point of indeterminate switching. More
specifically, Hk is the total anisotropy of the ferromagnetic layers
in the SAF, which includes contributions from the intrinsic material
anisotropy Hki, and from shape anisotropy Hks, so that
Hk=Hki+Hks. For reliable toggle switching, the vector sum
of the applied field pulses should be at least Hsw and less than
Hsat. The difference between Hsat and Hsw is defined as
the operating window and is preferably large enough to prevent errors.
Lower Hsw is desirable for realizing low power devices. One way to
reduce Hsw is by reducing Hsat as Hsw= {square root over
(HkHsat)}. However, this approach shrinks the operating window
because Hsw α Hsat, especially for high Hk
materials. Also, SAFs are known to be temperature dependant. That is,
their magnetic properties are strongly dependent upon the ambient thermal
environment, which limits the range of temperatures at which the device
may operate. For example, the saturation field, Hsat, of a NiFe SAF
measured at temperature typically drops, as temperature is increased, at
a rate of about 0.4%/° C. (defined as temperature-coefficient
(TC)). This drop, though reversible, leads to a reduced operating window
at elevated temperature as the Hsat drops faster than Hsw
(since Hsw α Hsat).

[0006]Free-layer ferromagnetic materials that give rise to high
magnetoresistance (MR) due to their large spin polarization, such as
NiFeCo and CoFeB, generally have high intrinsic Hki. Hereinafter,
the term "anisotropy field" refers to the intrinsic anisotropy Hki.
However, for standard toggle MRAM free layers, such ferromagnetic
materials with high Hki lead to high switching field and a small
operating window for the same Hsat.

[0007]Many conventional SAF structures used in toggle MRAMs do not have a
wide enough operating window for operation in large operating temperature
ranges, such as those present in automotive applications. This has
prompted the use of multilayer SAFs (e.g., four-layer SAFs), wherein the
Hsat and Hsw can be controlled independently. However, even
such multilayer structures, as found by the inventors, are known to
exhibit significant temperature dependence.

[0008]The multilayer SAF structure is one example of an exchange-coupled
magnetic multilayer structure. In the multilayer SAF structure, the
thickness of the coupling layers is adjusted to provide antiferromagnetic
coupling between the adjacent ferromagnetic layers. For some magnetic
devices, including some MRAM free layer structures, it is desirable to
have the thickness of one or more of the coupling layers adjusted to
provide ferromagnetic coupling.

[0009]It is therefore desirable to provide improved exchange-coupled
magnetic multilayer structures for MRAM devices that exhibit low power
while offering a wide operating temperature range. Other desirable
features and characteristics of the present invention will become
apparent from the subsequent detailed description of the invention and
the appended claims, taken in conjunction with the accompanying drawings
and this background of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]The present invention will hereinafter be described in conjunction
with the following drawing figures, wherein like numerals denote like
elements, and

[0013]FIG. 3 is a conceptual cross-sectional view of a SAF in accordance
with one embodiment.

DETAILED DESCRIPTION

[0014]In general, what is described herein are methods and apparatus for a
magnetic tunnel junction (MTJ) comprising a synthetic antiferromagnet
(SAF) structure formed on a tunnel barrier layer, wherein the SAF
includes a plurality (e.g., four or more) ferromagnetic (FM) layers
antiferromagnetically or ferromagnetically coupled through a plurality of
respective coupling (or "spacer") layers comprising, for example, Ru. The
ferromagnetic layers or the coupling layers are treated to reduce their
microcrystalline texture, thereby improving the operating window and
temperature range of the SAF. A measure of the amount of microcrystalline
texture can be obtained from rocking curves made by varying the sample
angle while holding the detector angle fixed on a peak identified in a
θ-2θ x-ray diffraction spectrum. Microcrystalline texture is
characterized by the full-width-at-half-maximum (FWHM) of the peak
obtained from the x-ray rocking curve measurement, which represents the
angular distribution of the crystallite orientations present in the
material. In one embodiment, the FM layers exhibit a microcrystalline
texture characterized by a rocking curve FWHM of greater than
approximately 15°.

[0015]The following detailed description is merely exemplary in nature and
is not intended to limit the range of possible embodiments and
applications. Furthermore, there is no intention to be bound by any
theory presented in the preceding background or the following detailed
description.

[0016]For simplicity and clarity of illustration, the drawing figures
depict the general structure and/or manner of construction of the various
embodiments. Descriptions and details of well-known features and
techniques may be omitted to avoid unnecessarily obscuring other
features. Elements in the drawing figures are not necessarily drawn to
scale: the dimensions of some features may be exaggerated relative to
other elements to assist improve understanding of the example
embodiments.

[0017]Terms of enumeration such as "first," "second," "third," and the
like may be used for distinguishing between similar elements and not
necessarily for describing a particular spatial or chronological order.
These terms, so used, are interchangeable under appropriate
circumstances. The embodiments of the invention described herein are, for
example, capable of use in sequences other than those illustrated or
otherwise described herein. Unless expressly stated otherwise,
"connected" means that one element/node/feature is directly joined to (or
directly communicates with) another element/node/feature, and not
necessarily mechanically. Likewise, unless expressly stated otherwise,
"coupled" means that one element/node/feature is directly or indirectly
joined to (or directly or indirectly communicates with) another
element/node/feature, and not necessarily mechanically.

[0018]The terms "comprise," "include," "have" and any variations thereof
are used synonymously to denote non-exclusive inclusion. The terms
"left," right," "in," "out," "front," "back," "up," "down," and other
such directional terms are used to describe relative positions, not
necessarily absolute positions in space. The term "exemplary" is used in
the sense of "example," rather than "ideal."

[0019]In the interest of conciseness, conventional techniques, structures,
and principles known by those skilled in the art may not be described
herein, including, for example, standard MRAM processing techniques,
fundamental principles of magnetism, and basic operational principles of
memory devices. For the purposes of clarity, some commonly-used layers
may not be illustrated in the drawings, including various protective cap
layers, seed layers, and the underlying substrate (which may be a
conventional semiconductor substrate or any other suitable structure).

[0020]MTJs in accordance with various embodiments may include any number
of ferromagnetic layers, and may be incorporated into a variety of
structures, such as toggle MRAM, hard disk drive and magnetic sensors and
the like. FIG. 3 depicts a SAF structure 300 formed on a tunnel barrier
layer 108 in accordance with one embodiment. SAF 300 in this embodiment
includes four ferromagnetic layers (i.e., four ferromagnetic layers 302,
306, 310, and 314) separated and antiferromagnetically coupled to each
other via respective coupling layers 304, 308, and 312, wherein the
bottommost ferromagnetic layer 314 is formed adjacent to tunneling
barrier (or "tunnel barrier") 108. The magnitudes of the
antiferromagnetic coupling for each pair can be adjusted by adjusting the
layers 304, 308 and 312. In some cases it is desirable to adjust some
layers to provide ferromagnetic coupling, for example, 304 and 312 can be
adjusted for ferromagnetic coupling while the others are adjusted for
antiferromagnetic coupling.

[0021]While the entire structure of FIG. 3 may be referred to as a SAF, it
will be appreciated that the illustrated structure may be characterized
as including multiple SAFs--i.e., one SAF comprising layers 310, 312, and
314, and another SAF comprising layers 302, 304, and 306. These two SAFs,
often referred to as the outer SAFs, are
antiferromagnetically/ferromagnetically coupled to each other via middle
coupling layer 308. The SAF comprising layers 306, 308, and 310 is
referred to as the center SAF. Thus, structure 300 is alternatively
referred to as a multilayer-SAF, or "ML-SAF."

[0022]In accordance with various embodiments, the MLs and/or coupling
layers within structure 300 exhibit a reduced or substantially zero
microcrystalline texture (e.g., an x-ray rocking curve FWHM measurement
of greater than 10° and preferably greater than 15°), which
may also be referred to as a "weak" texture. That is, as it will be
understood that the stack shown in FIG. 3 is deposited in a series of
layers, starting at 108, and ending with 302, various surfaces are
exposed prior to subsequent processing (e.g., surfaces 332, 330, 328,
326, 324, 322, and 320). These surfaces may be subjected to various
processing steps to reduce microcrystalline texture of
subsequently-formed layers.

[0023]The present inventors have found that the increased TCs of Hsat
and Hsw in multilayer SAFs such as those shown in FIG. 3 is due in
part to the increase microcrystalline texture of the upper FM layers
(e.g., layers 302 and 306). The first FM layer (314) deposited on the
amorphous tunnel barrier (e.g., Aluminum Oxide) is quite disordered;
however, the microcrystalline texture becomes more pronounced in the
later grown FM layers (310, 306 and 302). The increased texture in these
upper layers is primarily due to Ru, the coupling layer (312, 308 and
304), which the inventors have found to promote texture in FM layers. In
a 4-layer ML SAF as shown, reducing the texture of layers 302 and 306
results in a substantial improvement in TC.

[0024]The texture of the various layers may be reduced in a variety of
ways. In one embodiment, the texture of the crystalline-based ML SAF
layers is reduced by surface treatment (after deposition of the layer, or
intermittently)--for example, oxygen exposure (oxygen treatment) for a
short duration (5-20 seconds) to the spacer layers (304, 308, 312), after
deposition and/or leaking a small amount of O2 or N2 during
fabrication of FM layers (302, 306, 310, 314) or coupling layers (304,
308, 312). In one embodiment, for example, a NiFe-based ML SAF, wherein
the second and third spacers 308 and 304 comprise Ru, are exposed to
oxygen for a short duration, typically around 10 seconds. The Ru spacer
may be surface treated or doped--for example, with oxygen or nitrogen.

[0025]The use of amorphous layers for layers 302, 306, 310, and 314 may
also be used to further reduce the texture of these layers. In one
embodiment, for example, CoFeB is used for one or more of these layers,
where B content is more than 9 atomic percent.

[0026]In yet another embodiment, thin layers that are known to reduce the
texture of layers grown above them (i.e., "detexturing layers") can be
used--e.g., Aluminum. The inventors have found that Ru (the preferred
antiferromagnetic coupling layer) promotes texturing of the ferromagnetic
layers grown above it. Growing a thin layer of Al, for example, in the
middle of the layer 306 disrupts the texture propagation through the
stack.

[0027]While FIG. 3 depicts a SAF 300 with four ferromagnetic layers, the
range of embodiments is not so limited. Any number of layers may be
formed in a particular embodiment. That is, in general, SAF 300 may have
N FM layers and N-1 spacer layers, where N-2 FM layers exhibit reduced or
substantially zero crystalline texture. In one embodiment, for example,
the topmost N-2 FM layers are thus detextured.

[0028]As mentioned before, ML SAF structures are proposed as a solution to
the operating window issues exhibit significant temperature dependence.
In fact, the inventors have found that these ML structures have even less
desirable temperature dependence than the conventional toggle MRAM
structures. For example, NiFe-based ML structures exhibit a more
significant operating temperature dependence of their saturation field,
Hsat(T) (0.46%/° C. vs. 0.4%/° C.) as well as
switching field, Hsw(T) (0.46%/° C. vs. 0.32%/° C.)
compared to conventional NiFe-based toggle MRAM free layer. As the
saturation field is high (which is controlled by the inner SAF), a high
Hsat(T) is not a major issue; however, higher Hsw(T)
(controlled by the outer SAFs) can be a significant problem. To address
the High Hsw(T), it is necessary to raise the temperature
compensation, also known as temperature coefficient (TC), built into the
circuit to compensate for the change in Hsw with temperature. This
is undesirable as it leads to a large change in Hsw or switching
current over the desired operating temperature range.

[0029]In addition to the reduced texturing approach, another embodiment
designed to improve the poor operating temperature behavior of ML SAFs is
a ML SAF wherein the Hsat of the outer SAF having a higher TC is
maintained at a value greater than the Hsat of the outer SAF with a
lower TC over the desired temperature range. The outer SAF with the
lowest Hsat then determines the TC of the switching field for the
entire structure; in other words, if the Hsat of the outer SAF with
lower TC can be maintained lower than the other outer SAFs (having a
higher TC) over the desired temperature range, then the entire ML
structure will exhibit a better TC of Hsw. The preferred structure
is a NiFe ML SAF, wherein the process starts with a high Hsat for
the outer SAF with higher TC (which is the top SAF in the illustrated
embodiment). However, the Hsat is not so high that Hsat
differences cause the two outer SAFs to switch independently (i.e., if
the two outer SAFs become independent, then the advantage of ML SAF
configuration is lost). Such a structure improves the at-temperature
behavior of the preferred ML SAF.

[0030]While at least one exemplary embodiment has been presented in the
foregoing detailed description, it should be appreciated that a vast
number of variations exist. It should also be appreciated that the
exemplary embodiment or exemplary embodiments are only examples, and are
not intended to limit the scope, applicability, or configuration of the
embodiments in any way. Rather, the foregoing detailed description will
provide those skilled in the art with a convenient road map for
implementing an exemplary embodiment, it being understood that various
changes may be made in the function and arrangement of elements described
in an exemplary embodiment without departing from the scope as set forth
in the appended claims.